The housing of a conventional meter for measuring electric energy consumption, for example, typically comprises a base and a cylindrical cover secured to the base and enclosing the internal meter components. The cover typically includes a series of circumferentially spaced apart clips for securing the cover to the base. Attachment of the cover to the base is accomplished by positioning the cover on top of the base, aligning the clips with corresponding slots in a flange on the base, and then rotating the cover relative to the base. Rotation of the cover causes the clips to grip the base flange and deflect slightly, producing a torque of between 2 and 10 foot-pounds in cooperation with a sealing gasket positioned between cover and base flange.
The desired seating torque range for securing a conventional meter cover to its base is 2 to 6 foot-pounds. When the torque exceeds this range, the cover may become difficult to remove by hand, and a special tool may be required for removal. Additionally, the cover clips may break if the torque range is exceeded by rotating the cover too far.
When the seating torque falls below the desired range, such as if the cover is not rotated far enough relative to the base, the cover may become disengaged from the meter base during shipping and handling. A low seating torque is also unacceptable because a meter is often handled by its cover while the base carries the major components, and, hence, weight of the meter. For example, in removing a meter from a socket, 30 to 120 pounds of force may be placed on the cover.
The service environment of most electric meters presents additional problems related to the seating torque of the base and cover. Conventional meter bases are manufactured from a thermosetting phenolic resin which is susceptible to both shrinkage and swelling. Because phenolic resins are hygroscopic, humidity and other moisture may cause the base to swell by as much as 3%, resulting in an unacceptably high torque which leads to difficulty in removing the cover or in causing permanent deformation of the plastic cover clips. Furthermore, exposure to high temperatures can cause the base to shrink by as much as 3%, resulting in unacceptably low or no seating torque which may cause the cover to become loose relative to the base during service. Accordingly, the selection of materials and the proper initial assembly seating torque are important for a long meter service life.